Recently, there has been increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. More recently, in the development of such batteries, active studies have been conducted to design a novel electrode and battery, which provide an improved capacity density and specific energy.
A secondary battery comprises a cathode, anode and an electrolyte. Such secondary batteries are capable of repeated charge/discharge cycles, because lithium ions deintercalated from a cathode active material upon the first charge cycle are intercalated into an anode active material (for example, carbon particles) and deintercalated again during a discharge cycle, so that lithium ions reciprocate between both electrodes while transferring energy.
Meanwhile, due to high reactivity of a lithium ion during initial charge of a secondary battery, the lithium ion, an electrolyte solvent, an anode active material, etc. may form a sort of SEI (Solid Electrolyte Interface) layer on the surface of the anode active material through a reaction. The SEI layer may prevent decomposition of an electrolyte at an anode surface during charge/discharge of a battery, or a structural collapse of an electrode, which is caused by the co-intercalation of an electrolyte solvent into an anode active material. However, an SEI layer formed by a conventional electrolyte solvent, for example, a carbonate-based organic solvent, is usually weak and porous, and thus is not enough to perform a role of a continuous protective layer for an anode. Especially, the SEI layer is not thermally stable, and thus is subject to break-down by electrochemical and thermal energy increased with the passage of time when a battery is driven or left at high temperatures. Accordingly, at high temperatures, the SEI layer may be re-formed, thereby reducing battery capacity. Also, a side reaction such as electrolyte decomposition may occur on an anode surface exposed by collapse of the SEI layer, and thus gas such as CO2 may be generated, thereby causing a swelling phenomenon, that is, a thickness increase of a battery. This may cause a problem of safety deterioration in a product set employing a battery (such as a cellular phone, a notebook, etc.).